JP7055476B2 - Positive electrode and secondary battery containing the positive electrode - Google Patents

Description

[Cross-reference of related applications]
This application has priority based on Korean Patent Application No. 10-2018-0019487 filed on February 19, 2018 and Korean Patent Application No. 10-2019-0018735 filed on February 18, 2019. All the contents disclosed in the literature of the Korean patent application claiming the interest of the above are included as a part of the present specification.

The present invention relates to a positive electrode that satisfies the optimum relationship between the content of a specific binder and the content of a specific carbon nanotube, and a secondary battery containing the same.

Recently, with the development of technology for mobile devices and the increase in demand, the demand for batteries as an energy source has increased rapidly, and as a result, various studies have been conducted on batteries that can meet various demands. In particular, as a power source for such an apparatus, research on a lithium secondary battery having a high energy density and an excellent life and cycle characteristics is being actively promoted.

The lithium secondary battery includes a positive electrode containing a positive electrode active material capable of inserting / removing lithium ions, a negative electrode containing a negative electrode active material capable of inserting / removing lithium ions, and the positive electrode and the negative electrode. It means an electrode assembly containing a fine porous separation film interposed between the two, and a battery containing a non-aqueous electrolyte containing lithium ions.

In the positive electrode and / or the negative electrode, the positive electrode active material layer may contain a conductive material in order to improve the conductivity. Conventionally, point-type conductive materials such as carbon black have been mainly used. However, if the content of the conductive material is increased in order to improve the conductivity, the amount of the positive electrode active material or the negative electrode active material is relatively reduced, the capacity of the battery is reduced, or the positive electrode binder or the negative electrode binder is reduced. There is a problem that the adhesive strength drops. In particular, in the case of a positive electrode, the conductivity of the positive electrode active material itself is at a low level, so that the problem becomes even greater.

In order to solve this, a method using a linear conductive material such as carbon nanotubes has been introduced. Since the carbon nanotubes have a length relatively longer than that of the particle-type conductive material, even a small amount can improve the conductivity and the bonding force of the positive electrode active material layer constituent material.

On the other hand, in order to improve the adhesive force (positive electrode adhesive force) between the positive electrode active material layer and the current collector, the positive electrode active material layer may contain a binder. The binder generally does not contribute to the improvement of the conductivity of the positive electrode active material layer. As a result, even if carbon nanotubes are used, there is a limit to the improvement of conductivity when used in combination with a binder.

Therefore, research is underway to improve the characteristics of binders and carbon nanotubes.

An object of the present invention is to maintain the viscosity of the positive electrode slurry at a preferable level, which is advantageous in terms of processability and manufacturing cost, and has good positive electrode adhesive strength and improved conductivity. The purpose is to provide a secondary battery including this.

According to one embodiment of the present invention, the current collector and the positive electrode active material layer arranged on the current collector are included, and the positive electrode active material layer contains a positive electrode active material, carbon nanotubes, and a binder. The binder contains a polyvinylidene fluoride having a weight average molecular weight of 720,000 g / mol to 980,000 g / mol, and the BET specific surface area of the carbon nanotube is 140 m ² / g to 195 m ² / g, according to the following formula 1. A positive electrode that meets the requirements is provided.
[Equation 1]
1.3 ≤ B / A ≤ 3.4
In the formula 1, B is the content (% by weight) of the polyvinylidene fluoride in the positive electrode active material layer, and A is the content (% by weight) of the carbon nanotube in the positive electrode active material layer. ..

According to another embodiment of the present invention, a secondary battery including the positive electrode is provided.

According to the present invention, the conductivity of the positive electrode and the adhesive strength of the positive electrode are improved by adjusting the BET specific surface area of the carbon nanotube, the content ratio of the polyvinylidene fluoride to the carbon nanotube, and the weight average molecular weight of the polyvinylidene fluoride. However, it is possible to suppress an excessive increase and an excessive decrease in the positive electrode slurry during the production of the positive electrode. This has advantages in terms of processability and manufacturing cost when manufacturing the positive electrode, the positive electrode slurry can be easily applied, and the positive electrode active material layer can be uniformly formed. In addition, the output of the battery and the life characteristics at high temperature can be further improved.

Hereinafter, the present invention will be described in more detail in order to help the understanding of the present invention. At this time, the terms and words used in the present specification and the scope of the patent claim should not be construed only in a normal and lexical meaning, and the inventor explains his invention in the best way. In order to do so, it must be interpreted as a meaning and concept that fits the technical idea of the present invention, in line with the principle that the concept of terms can be defined as appropriate.

The positive electrode according to one embodiment of the present invention includes a current collector and a positive electrode active material layer arranged on the current collector, and the positive electrode active material layer contains a positive electrode active material, carbon nanotubes, and a binder. The binder contains a polyvinylidene fluoride having a weight average molecular weight of 720,000 g / mol to 980,000 g / mol, and the BET specific surface area of the carbon nanotube is 140 m ² / g to 195 m ² / g, according to the following formula. Satisfy 1.
[Equation 1]
1.3 ≤ B / A ≤ 3.4
In the formula 1, B is the content (% by weight) of the polyvinylidene fluoride in the positive electrode active material layer, and A is the content (% by weight) of the carbon nanotube in the positive electrode active material layer. ..

The current collector may be any as long as it has conductivity without inducing a chemical change in the battery, and is not particularly limited. For example, as the current collector, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface is surface-treated with carbon, nickel, titanium, silver, or the like may be used. good. Specifically, a transition metal that adsorbs carbon well, such as copper and nickel, may be used as the current collector.

The positive electrode active material layer may be arranged on the current collector. The positive electrode active material layer may be arranged on one side or both sides of the current collector.

The positive electrode active material layer may contain a positive electrode active material, carbon nanotubes, and a binder. Specifically, the positive electrode active material layer may be composed of a positive electrode active material, carbon nanotubes, and a binder.

The positive electrode active material is Li [Ni _x1 Mn _{y1 Coz1} ] O ₂ (0.40 ≦ x1 ≦ 0.70, 0.15 ≦ y1 ≦ 0.30, 0.15 ≦ z1 ≦ 0.30, x1 + y1 + z1 = 1) may be included. Since the positive electrode active material has a high energy density, it enables the production of a high-capacity battery. Specifically, the positive electrode active material is Li (Ni _x2 Mn _y2 Co _z2 ) O ₂ (0.56 <x2 <0.68, 0.16 <y2 <0.22, 0.16 <z2 <0. .22, x2 + y2 + z2 = 1) may be included. The positive electrode active material has a high energy density but is excellent in safety.

The average particle size (D ₅₀ ) of the positive electrode active material may be 3 μm to 20 μm, specifically 6 μm to 18 μm, and more specifically 9 μm to 16 μm. When the above range is satisfied, the characteristics of the high temperature life and the output characteristics of the battery can be improved. The average particle size (D ₅₀ ) in the present specification can be defined by the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve of the particles. The average particle size (D ₅₀ ) can be measured, for example, by using a laser diffraction method. The laser diffraction method can generally measure a particle size of about several mm from a submicron region, and can obtain highly reproducible and highly decomposable results.

The positive electrode active material may be 95.6% by weight to 99.0% by weight, specifically 97.0% by weight to 98.0% by weight in the positive electrode active material layer. .. When the above range is satisfied, the adhesive force of the positive electrode and the conductivity of the positive electrode can be improved at the same time.

The carbon nanotubes in the present invention can play the role of a positive electrode conductive material. Specifically, the positive electrode active material layer may contain only carbon nanotubes as the positive electrode conductive material. For example, when the positive electrode active material layer contains a particulate conductive material such as acetylene black or a plate-shaped conductive material in addition to carbon nanotubes, the conductivity of the positive electrode may be lower than that of the positive electrode of the present invention.

The carbon nanotube may be a bundle type carbon nanotube. The bundle-type carbon nanotube may contain a plurality of carbon nanotube units. Specifically, the above-mentioned "bundle type" means that, unless otherwise mentioned, a plurality of carbon nanotube units are arranged side by side with substantially the same orientation in the longitudinal axis of the carbon nanotube units. Refers to a secondary shape in the form of a bundle or rope that is or is intertwined. The carbon nanotube unit has a graphite sheet in the form of a cylinder having a nano ^- sized diameter and has an sp2 bonding structure. At this time, the characteristics of the conductor or the semiconductor can be shown depending on the angle and structure at which the graphite sheet is wound. The carbon nanotube unit is a single-walled carbon nanotube (SWCNT, single-walled carbon nanotube) unit, a double-walled carbon nanotube (DWCNT, double-walled carbon nanotube) unit, and a multi-walled carbon, depending on the number of bonds forming the wall. It may be classified into nanotube (MWCNT, multi-walled carbon nanotube) units. Specifically, the carbon nanotube unit body may be a multilayer carbon nanotube unit body. The multi-walled carbon nanotube unit is preferable in that the energy required for dispersion is lower than that of the single-walled carbon nanotube unit and the double-walled carbon nanotube unit, and the dispersion conditions are at a level that is easy to adjust.

The average diameter of the carbon nanotube unit may be 1 nm to 30 nm, specifically 3 nm to 26 nm, and more specifically 5 nm to 22 nm. When the above range is satisfied, the carbon nanotubes are easily dispersed in the positive electrode slurry, and the conductivity of the positive electrode can be improved. The average diameter can be measured by TEM or SEM.

The BET specific surface area of the carbon nanotubes may be 140 m ² / g to 195 m ² / g, specifically 145 m ² / g to 195 m ² / g, and more specifically 160 m ² It may be from / g to 190 m ² / g. When the BET specific surface area of the carbon nanotubes is less than 140 m ² / g, the viscosity of the positive electrode slurry at the time of positive electrode production is too low, so that the positive electrode slurry coating and drying processability is deteriorated, and the production cost is excessively increased. .. Further, due to the decrease in the BET specific surface area, the conductive path is reduced, and the conductivity of the positive electrode is greatly reduced. On the other hand, when the BET specific surface area of the carbon nanotubes exceeds 195 m ² / g, the viscosity of the positive electrode slurry is too high, and it is very difficult to apply the positive electrode slurry to the current collector, and the positive electrode slurry is uniformly applied. Therefore, the formed positive electrode active material layer is not uniform. Further, due to the increase in the BET specific surface area, the dispersibility of the carbon nanotubes is lowered, and the conductivity of the positive electrode is greatly lowered. The BET specific surface area may be measured via the nitrogen adsorption BET method.

The binder may contain polyvinylidene fluoride (PVdF).

The weight average molecular weight of the polyvinylidene chloride may be 720,000 g / mol to 980,000 g / mol, specifically 750,000 g / mol to 950,000 g / mol, and more specifically. It may be 800,000 g / mol to 920,000 g / mol. When the weight average molecular weight is less than 720,000 g / mol, even if the formula 1 described later is satisfied, the viscosity of the positive electrode slurry formed at the time of manufacturing the positive electrode is too low, so that the positive electrode slurry coating is difficult and the step at the time of drying. The sex will be excessively reduced and the manufacturing cost will be excessively increased. In addition, since the positive electrode adhesive force is reduced too much, there arises a problem that the positive electrode active material is desorbed. On the other hand, when the weight average molecular weight exceeds 980,000 g / mol, there is a problem that the positive electrode and the battery resistance increase too much. In addition, the viscosity of the positive electrode slurry rises excessively, for example, excessively rises to 50,000 cp or more at room temperature, making it difficult to apply the positive electrode slurry itself, and the formed positive electrode active material layer becomes uneven, resulting in poor battery performance. Can be reduced. The weight average molecular weight of the polyvinylidene fluoride, when considering the amount of the carbon nanotubes used, improves the processability and manufacturing cost, improves the uniformity of the positive electrode active material layer, and improves the conductivity of the positive electrode within the limited composition. , And the optimum range for simultaneously satisfying the improvement of the positive electrode adhesive force.

The binder may further contain a non-fluorine-based binder. The non-fluorine-based binder may be at least one of a nitrile butadiene rubber (Nitrile butyldie rubber; NBR) and a hydrogenated nitrile butadiene rubber (Hydrogenated-Nitrile buddiene rubber; H-NBR), and specifically. May be hydrogenated nitrile butadiene rubber.

The weight ratio of the polyvinylidene chloride to the non-fluorinated binder may be 23: 1 to 1: 1 or, specifically, 20: 1 to 3: 1. When the above range is satisfied, there are effects of improving the adhesive force of the positive electrode and improving the dispersion of carbon nanotubes.

The positive electrode of the present invention can satisfy the following formula 1.

[Equation 1]
1.3 ≤ B / A ≤ 3.4
In the formula 1, B is the content (% by weight) of the polyvinylidene fluoride in the positive electrode active material layer, and A is the content (% by weight) of the carbon nanotube in the positive electrode active material layer. ..

When the B / A is less than 1.3, the positive electrode adhesive force is too weak and the positive electrode active material is desorbed during the production of the positive electrode. Further, the viscosity of the positive electrode slurry rises too much, it is difficult to apply the positive electrode slurry itself, the formed positive electrode active material layer becomes uneven, and the battery performance may deteriorate. On the other hand, when the B / A exceeds 3.4, the positive electrode adhesive strength is excellent, but the powder resistance is too high, so that the battery resistance increases excessively. As a result, the output characteristics of the battery are greatly reduced. Further, since the viscosity of the positive electrode slurry formed at the time of manufacturing the positive electrode is too low, it is difficult to coat the positive electrode slurry, the processability at the time of drying is excessively lowered, and the manufacturing cost is excessively increased.

Specifically, the positive electrode can satisfy the following formula 2.

[Equation 2]
1.3 ≤ B / A ≤ 1.85

When the above formula 2 is satisfied, the powder resistance and the positive electrode adhesive force can be further improved. This is because the viscosity of the positive electrode slurry increases to some extent to a level that does not impair the uniformity of the positive electrode active material layer, while the migration of the binder in the positive electrode slurry is suppressed. As a result, the adhesive force of the positive electrode is improved and the binder is not applied non-uniformly, so that the conductivity of the entire positive electrode can be further improved.

The secondary battery according to still another embodiment of the present invention may include a positive electrode; a negative electrode; a separation film interposed between the positive electrode and the negative electrode; and an electrolyte, and the positive electrode is the positive electrode of the above-described embodiment. Is the same as. Therefore, the description of the positive electrode will be omitted.

The negative electrode may include a negative electrode current collector and a negative electrode active material layer arranged on one or both sides of the negative electrode current collector.

The negative electrode current collector may be any as long as it has conductivity without inducing a chemical change in the battery, and is not particularly limited. For example, as the negative electrode current collector, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or a surface-treated aluminum or stainless steel surface with carbon, nickel, titanium, silver, or the like is used. May be good. Specifically, a transition metal that adsorbs carbon well, such as copper and nickel, may be used as the current collector.

The negative electrode active material layer may contain a negative electrode active material, a negative electrode conductive material, and a negative electrode binder.

The negative electrode active material may be graphite-based active material particles or silicon-based active material particles. As the graphite-based active material particles, one or more selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fiber and graphitized mesocarbon microbeads can be used, and in particular, when artificial graphite is used, the rate can be used. The characteristics can be improved. The silicon-based active material particles are Si, SiO _x (0 <x <2), Si—C composite, and Si—Y alloy (where Y is an alkali metal, an alkaline earth metal, a transition metal, and a group 13 element. , 14 group elements, rare earth elements and elements selected from the group consisting of combinations thereof) can be used at least one selected from the group, and particularly when Si is used, a high capacity of the battery is derived. can do.

The negative electrode binders include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (polyvinylidenefluoride), polyacrylonitrile, polymethylmethacrylate (polymethylmethacryllate), and polyvinyl alcohol (polyacrylic acid). ), Distillate, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, It may contain at least one selected from the group consisting of polyacrylic acid and substances in which these hydrogens are substituted with Li, Na, Ca or the like, and various copolymers thereof may be contained. It may be included.

The negative electrode conductive material is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, graphite such as natural graphite or artificial graphite; carbon black, acetylene black, etc. Carbon black such as Ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fiber and metal fiber; conductive tubes such as carbon nanotubes; metal powder such as fluorocarbon, aluminum and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used.

The separation film separates the negative electrode and the positive electrode to provide a transfer passage for lithium ions, and can be used without particular limitation as long as it is usually used for the separation film in a secondary battery, and in particular, it can be used. Those having low resistance to ion transfer of the electrolyte and excellent in electrolyte impregnation ability are preferable. Specifically, polyolefin-based high polymers such as porous polymer films such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers and ethylene / methacrylate copolymers. A porous polymer film made of molecules or a laminated structure having two or more layers thereof may be used. Further, a normal porous non-woven fabric, for example, a non-woven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, or the like may be used. Further, in order to secure heat resistance or mechanical strength, a coated separation membrane containing a ceramic component or a polymer substance may be used, or may be selectively used in a single-layer or multilayer structure. ..

Examples of the electrolyte include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-like polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte that can be used in the manufacture of a lithium secondary battery. Not limited to.

Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran (franc), 2 -Methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3 An aprotic organic solvent such as -dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, ethyl propionate and the like may be used.

In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are highly viscous organic solvents and may be suitably used because they have a high dielectric constant and dissociate lithium salts well. When a linear carbonate having a low viscosity and a low dielectric constant such as dimethyl carbonate and diethyl carbonate is mixed at an appropriate ratio with the cyclic carbonate, an electrolyte having high electric conductivity can be produced. It can be used more preferably.

A lithium salt can be used as the metal salt, and the lithium salt is a substance that is easily dissolved in the non-aqueous electrolytic solution. For example, the negative ions of the lithium salt include F ⁻ , Cl ⁻ , and I. ^- , NO _3- ^, N (CN) _2- ^, BF _4- ^, ClO _4- ^, PF _6- ^, (CF ₃ ) ₂ PF _4- ^, (CF ₃ ) ₃ PF _3- ^, (CF ₃ ) ₄ PF ₂ ^- , (CF ₃ ) ₅ PF- ^, (CF ₃ ) ₆ P- ^, CF ₃ SO _3- ^, CF ₃ CF ₂ SO _3- ^, (CF ₃ SO ₂ ) ₂ N- ^, (FSO ₂ ) ₂ N- ^, CF ₃ CF ₂ (CF ₃ ) ₂ CO- ^, (CF ₃ SO ₂ ) ₂ CH- ^, (SF ₅ ) ₃ C- ^, (CF ₃ SO ₂ ) ₃ C- ^, CF ₃ (CF ₂ ⁾ ₇ SO _3- , CF ₃ CO _2- ^, CH ₃ CO _2- ^, SCN- ^and (CF ₃ CF ₂ SO ₂ ⁾ ₂ N-.

In addition to the constituent components of the electrolyte, the electrolyte is a haloalkylene carbonate type such as difluoroethylene carbonate for the purpose of improving the life characteristics of the battery, suppressing the decrease in the battery capacity, and improving the discharge capacity of the battery. Compound, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivative, sulfur, quinoneimine dye, N-substituted oxazolidinone, N, N-substituted imidazolidine, ethylene. One or more additives such as glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol or aluminum trichloride may be further contained.

According to still another embodiment of the present invention, there is provided a battery module containing the secondary battery as a unit cell and a battery pack containing the same. Since the battery module and battery pack include the secondary battery having high capacity, high rate characteristics and cycle characteristics, it is selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and power storage systems. It may be used as a power source for medium and large-sized devices.

Hereinafter, examples of the present invention will be described in detail so that a person having ordinary knowledge in the technical field to which the present invention belongs can easily carry out the present invention. However, the present invention may be embodied in several different forms and is not limited to the examples described herein.

Example 1: Production of Positive Electrode Slurry Li (Ni _0.6 Mn _0.2 Co _0.2 ) O ₂ having an average particle size (D ₅₀ ) of 12 μm is used as the positive electrode active material, and carbon nanotubes are used as the conductive material. Bundled carbon nanotubes (specific surface area of 185 m ² / g) made of units (multi-walled) were used, and the average diameter of the carbon nanotube units was 12 nm.

A conductive material dispersion liquid containing the multilayer carbon nanotube, hydrogenated nitrile butadiene rubber (H-NBR), and N-methylpyrrolidone (NMP, N-methylpyrrolidone) as a dispersion medium was prepared. Then, by mixing the positive electrode active material, PVdF having a weight average molecular weight of 880,000 g / mol, the conductive material dispersion liquid, and NMP, the solid content is 72%, and the positive electrode active material, carbon nanotubes, and the like. A positive electrode slurry having a weight ratio of PVdF and H-NBR of 97.5: 0.7: 1.66: 0.14 was produced.

Examples 2 to 4 and Comparative Examples 1 to 7: Production of Positive Electrode Slurry The contents of the carbon nanotubes, PVdF and H-NBR, the BET specific surface area of the carbon nanotubes, and the weight average molecular weight of PVdF are shown in Table 1 below. The positive electrode slurry of Examples 2 to 4 and Comparative Examples 1 to 7 was produced by the same method as in Example 1 except that the changes were made as described above.

The contents of each of the positive electrode active material, carbon nanotubes, PVdF, and H-NBR are based on the total weight of the positive electrode active material layer.

Experimental Example 1: Evaluation of Viscosity of Positive Electrode Slurry The viscosities of the positive electrode slurries of Examples 1 to 4 and Comparative Examples 1 to 7 were measured at 25 ° C. using a Brookfield B-type viscometer. Specifically, the viscosity after rotating for 1 minute at a rotor number 64 and a rotation speed of 12 rpm was measured and shown in Table 2.

Experimental Example 2: Evaluation of Powder Resistance Each of the positive electrode slurry of Examples 1 to 4 and Comparative Examples 1 to 7 was vacuum dried at a temperature of 130 ° C. for 3 hours and then pulverized to produce a powder. Then, using the Loresta GP equipment of Mitsubishi Chem Analytic Co., Ltd., the pellets were manufactured under the condition of a load of 9.8 MPa in an atmosphere of 25 ° C. and a relative humidity of 50%. Then, after measuring the powder resistance by the 4-probe method, this is shown in Table 2.

Experimental Example 3: Evaluation of Positive Electrode Adhesive Strength A positive electrode was produced using each of the positive electrode slurries of Examples 1 to 4 and Comparative Examples 1 to 7. Specifically, the positive electrode slurry was applied to a positive electrode current collector (Al) having a thickness of 20 μm, and dried in a vacuum oven at 130 ° C. for 6 hours. Then, the current collector coated with the positive electrode slurry was placed between rolls heated to 60 ° C. and rolled at a pressure of 10 MPa, and the final thickness (current collector + active material layer) was 95 μm. A positive electrode having a loading amount of the positive electrode active material layer of 680 mg / 25 cm ² was produced.

The positive electrode was punched out to a size of 20 mm × 150 mm, fixed to the central portion of a 25 mm × 75 mm slide glass with a tape, and then the peel strength was measured at 90 degrees while peeling off the current collector using a UTM. After measuring 5 or more peel strengths for each positive electrode, the average value was shown in the positive electrode adhesive strength. The measurement results are shown in Table 2.

As shown in Table 2, in the cases of Examples 1 to 4 satisfying 1.3 ≦ B / A ≦ 3.4, good positive electrode adhesive strength is maintained even though the powder resistance is at a low level. You can see that there is. Further, since the viscosity of the positive electrode slurry is not at an excessively high level or a low level, it can be seen that problems such as processability do not occur. On the other hand, in the case of Comparative Example 1 in which the B / A (polyvinylidene chloride content / carbon nanotube content) is 0.97 and less than 1.3, the powder resistance is lower, but the positive electrode adhesive strength is higher. It turns out that it is too low. It can also be seen that the viscosity of the positive electrode slurry is excessively high. In the case of Comparative Example 2 in which the B / A is 6.9 and exceeds 3.4, the positive electrode adhesive force is very high, but the powder resistance is too high. That is, it can be confirmed that there is room for the positive electrode conductivity, the positive electrode adhesive force, and the viscosity of the positive electrode slurry to reach the preferable levels only after the preferable range of B / A is satisfied.

On the other hand, in the case of Comparative Example 3 and Comparative Example 7 in which the BET specific surface areas are 280 m ² / g and 220 m ² / g, respectively, which are higher than those in the examples, the viscosity of the positive electrode slurry is too high. In the case of Comparative Example 4 in which the BET specific surface area is 120 m ² / g, which is lower than that of the examples, it can be seen that the viscosity of the positive electrode slurry is too low. Therefore, since the viscosity of the positive electrode slurry can meet the preferable level only after the BET specific surface area of an appropriate level is satisfied, the coating and drying processability at the time of manufacturing the positive electrode is improved, the manufacturing cost is reduced, and the positive electrode activity is activated. The material layer can be formed uniformly. Further, since the powder resistance is too high in both Comparative Example 3 and Comparative Example 4, it can be seen that the conductivity of the positive electrode is ensured only after the BET specific surface area satisfies an appropriate level.

In addition, in the case of Comparative Example 5 in which the weight average molecular weight of polyvinylidene fluoride is 1,200,000 g / mol and is higher than 980,000 g / mol, both the powder resistance and the positive electrode slurry viscosity are at very high levels. You can see that. In the case of Comparative Example 6 in which the weight average molecular weight of polyvinylidene fluoride is 500,000 g / mol and is lower than 720,000 g / mol, the powder resistance is good, but the viscosity of the positive electrode slurry is too low, so that during the production of the positive electrode. The coating and drying process of the above material is extremely impaired, and the manufacturing cost has to increase. Moreover, since the positive electrode adhesive force is too low, desorption of the positive electrode active material is expected.

Claims (10)

The current collector and the positive electrode active material layer arranged on the current collector are included.
The positive electrode active material layer contains a positive electrode active material, carbon nanotubes, and a binder.
The binder comprises polyvinylidene chloride having a weight average molecular weight of 720,000 to 980,000 .
The BET specific surface area of the carbon nanotubes is 140 m ² / g to 195 m ² / g.
Positive electrode satisfying the following formula 1:
[Equation 1]
1.3 ≤ B / A ≤ 3.4
In the formula 1, B is the content (% by weight) of the polyvinylidene fluoride in the positive electrode active material layer, and A is the content (% by weight) of the carbon nanotube in the positive electrode active material layer. .. The positive electrode according to claim 1, wherein the carbon nanotube is a bundle-type carbon nanotube containing a plurality of carbon nanotube units. The positive electrode according to claim 2, wherein the carbon nanotube unit has an average diameter of 1 nm to 30 nm. The positive electrode according to any one of claims 1 to 3, which satisfies the following formula 2.
[Equation 2]
1.3 ≤ B / A ≤ 1.85
In the formula 2, B is the content (% by weight) of the polyvinylidene fluoride in the positive electrode active material layer, and A is the content (% by weight) of the carbon nanotube in the positive electrode active material layer. .. The positive electrode active material is Li [Ni _x1 Mn _{y1 Coz1} ] O ₂ (0.40 ≦ x1 ≦ 0.70, 0.15 ≦ y1 ≦ 0.30, 0.15 ≦ z1 ≦ 0.30, x1 + y1 + z1 = The positive electrode according to any one of claims 1 to 4, which comprises 1). The positive electrode according to any one of claims 1 to 5, wherein the positive electrode active material is contained in the positive electrode active material layer in an amount of 95.6% by weight to 99.0% by weight. The positive electrode according to any one of claims 1 to 6, wherein the average particle size (D ₅₀ ) of the positive electrode active material is 3 μm to 20 μm. The positive electrode according to any one of claims 1 to 7, wherein the binder further contains a non-fluorine-based binder. The positive electrode according to claim 8, wherein the non-fluorine-based binder is at least one of a nitrile butadiene rubber and a hydrogenated nitrile butadiene rubber. The positive electrode according to any one of claims 1 to 9.
Negative electrode;
A separation membrane interposed between the positive electrode and the negative electrode; and a secondary battery containing an electrolyte.